I heard somewhere that the general rule for that is first figuring out how many cylinders you have. In your case, you have 4. Now that you know that, mulitply 4 by 20 (about the maximum amount of n2o you want to spray w/o doing internals and timing blah). 4 x 20 = 80. An 80 shot is a big ass shot so i'd stick to 50 or, if you have the $, 75 shot.